Semiconductor devices such as an LED or a power module have a structure in which a semiconductor element is joined onto a circuit layer formed of a conductive material.
In a power semiconductor element for high-power control which is used to control wind force power generation, electric vehicles, hybrid vehicles, and the like, the amount of heat generated is great, and thus, as a substrate mounting this power semiconductor element for high powder control, for example, an insulating circuit substrate including an insulating layer formed of a ceramic substrate of aluminum nitride (AlN), alumina (Al2O3), or the like and a circuit layer formed by disposing a metal having an excellent conductive property on one surface of this insulating layer (a substrate for a powder module) has been broadly used in the related art.
For example, a power module (a semiconductor device) described in Patent Document 1 has a structure in which a substrate for a power module having a circuit layer formed of a metal such as Al or Cu formed on one surface of a ceramic substrate (an insulating circuit substrate) and a semiconductor element that is joined onto this circuit layer are provided.
In addition, a power module including a metallic layer formed by disposing a metal having an excellent thermal conductive property on the other surface of the ceramic substrate is also provided. In addition, the power module has a constitution in which a heat sink is joined to the metallic layer of the substrate for a power module and heat generated in the semiconductor element is transmitted toward the substrate for a power module and diffused to the outside through the heat sink.
When an electronic component such as a semiconductor element is joined to a circuit layer or a heat sink is joined to a metallic layer, for example, a method in which a solder material is used as described in Patent Document 1 is broadly used.
In recent years, from the viewpoint of environmental protection, for example, a lead-free solder such as a Sn—Ag-based solder material, a Sn—In-based solder material, or a Sn—Ag—Cu-based solder material has become mainstream as a solder material used to join an electronic component such as a semiconductor element and a circuit layer or join a heat sink and a metallic layer.
Here, in a circuit layer and a metallic layer formed of aluminum or an aluminum alloy, a natural oxide film of aluminum is formed on the surface, and thus it has been difficult to favorably join the circuit layer and the metallic layer to a semiconductor element and a heat sink using a solder material.
In addition, in a circuit layer or a metallic layer formed of copper or a copper alloy, there has been a concern that a molten solder material and copper may react with each other, a component of the solder material may intrude into the inside of the circuit layer, and the characteristics of the circuit layer and the metallic layer may deteriorate.
Therefore, in the related art, a semiconductor element has been manufactured using a solder material after a Ni-plated film is formed on the surface of a circuit layer and a metallic layer as described in Patent Document 1.
In addition, as described in Patent Document 1, in a case in which a semiconductor element and a heat sink are joined together using a solder material, there is a concern that, when used in a high-temperature environment, some of the solder may melt and the joining reliability among the semiconductor element, the heat sink, and an insulating circuit substrate may degrade.
As a joining method in which no solder material is used, for example, Patent Document 2 proposes a technique for joining an electronic component such as a semiconductor element or a heat sink using an Ag nano-paste.
In addition, for example, Patent Document 3 and 4 propose techniques for joining an electronic component such as a semiconductor element or a heat sink using an oxide paste including metallic oxide particles and a reducing agent formed of an organic substance.
However, as described in Patent Document 2 to 4, in the case of joining an electronic component such as a semiconductor element using a metal paste or an oxide paste without using a solder material, a joint layer formed of a sintered body of this paste is formed to be thinner than the solder material, and thus stress during the loading of a heat cycle is likely to act on the electronic component such as the semiconductor element, and there has been a concern that the electronic component such as the semiconductor element may break. Similarly, when a joint layer formed between a metallic layer and a heat sink becomes thin, heat strain generated due to a difference in thermal expansion coefficient between an insulated circuit substrate and the heat sink acts on the insulated circuit substrate, and there has been a concern that cleavage may be generated in an insulating layer.
Therefore, for example, Patent Document 5 proposes a technique in which an Ag underlayer using a glass-containing Ag paste is formed on a circuit layer formed of aluminum or copper and then the circuit layer and a semiconductor element are joined together through the Ag paste. In this technique, the glass-containing Ag paste is applied and sintered on the surface of the circuit layer formed of aluminum or copper, whereby an oxide coating formed on the surface of the circuit layer is reacted with glass and removed so as to form an Ag underlayer, and the semiconductor element is joined onto the circuit layer on which the Ag underlayer is formed through the Ag joint layer formed of a sintered body of the Ag paste. Here, the Ag underlayer includes a glass layer formed by the reaction between the glass and the oxide coating on the circuit layer and an Ag layer formed on the glass layer. In this glass layer, conductive particles are dispersed, and due to these conductive particles, the conductive property of the glass layer is ensured.
According to the technique described in Patent Document 5, in this case, a sufficient thickness can be ensured by the Ag underlayer and a joint layer formed of the sintered body of the Ag paste and a silver oxide paste, and thus it becomes possible to suppress the breakage of the semiconductor element or the generation of cleavage in the insulating layer.